The basic structure of a semiconductor detector for ionizing radiation includes a high-resistivity semiconductor-crystal having anode and cathode electrodes. The anode and the cathode are biased to produce a strong electric field in the semiconductor crystal. A high energy photon which is oriented towards the semiconductor-detector has a certain probability of interacting with the semiconductor, which depends on its energy. This radiation-matter interaction may occur via photoelectric absorption, scattering such as Compton scattering or electron-positron pair production.
As a result of the above mentioned types of interactions between the photon and the semiconductor, the entire energy of the photon or part of it, in the case of Compton scattering, is converted to production of multiple electron-hole pairs in a form of a cloud of charge carriers. The negative charge carriers (electrons) and the positive charge carriers (holes) are caused to drift by the strong electric field in the semiconductor towards the anode and the cathode respectively.
In an ideal situation, all the electrons and holes produced in the cloud by the photon interaction, drift without loss towards the anode and cathode electrodes respectively. The drift of a charge carrier in an electric field induces in the electrodes an induced current, known as "displacement current". This current is proportional to the velocity of the charge carrier. Thus the integration of this current over the time span of the drift is proportional to the charge of the carrier and is monotonically dependent on the length of the trajectory of the charge-carrier.
In the ideal case an electron trajectory starts at the site of the photon interaction where the electron has been created and terminates at the anode, and the trajectory of the hole related to this electron (as part of the electron-hole pair) starts at the same point and terminates at the cathode. In this ideal case the total integration of the current induced on the anode and cathode will be equal to the electronic charge. Conventionally, as well known in the art, the current induced by the whole charge cloud is integrated by a Charge Sensitive Amplifier (CSA).
In the ideal case where all the electrons and holes arrive at the anode and cathode respectively, the charge integrated by the CSA connected to the anode is equal to the charge created by the photon, i.e. to Q=ne when n is the number of electron-hole pairs created by the photon, and e is the charge of one electron. Moreover, as the number n is exactly proportional to the energy of the photon, where the proportionality is a characteristic of the material, the charge induced is a true measure of the energy of the photon. Thus in the ideal case, the charge induced in the CSA, and thus the energy measured, is independent of the site of the photon interaction with the detector, i.e. it is not dependent on the distance of the interaction point from the anode. However, in practice, not all charge carriers reach their respective electrodes within the relevant integration time of the CSA.
For instance, in CdTe and CdZnTe the velocities of electrons are an order of magnitude higher than those of the holes in the same electric field. Therefore, whereas electrons drift towards the anode within a time much shorter than the "life-time", which is the typical time it takes an electron to be trapped and its trajectory to be terminated, holes are trapped on their way towards the cathode, and their trajectory stopped before they reach the cathode. Thus the induced charge is smaller than the ideal Q due to the fact that the trajectories of the holes are cut short by trapping of the holes. The resultant effect is "low-energy-tailing" due to this incomplete charge collection. This effect is dependent on the site of photon interaction. At a site near the cathode this effect will be minimal, since the holes have a small distance to travel to the cathode, whereas the incomplete charge collection problem is more severe if the photon interaction takes place nearer to the anode, since there the hole has a longer distance to travel towards the cathode.
In this situation a Multi-Channel Analyzer (MCA) spectrum, which is a graph of the number of counts vs. energy (channel), has a long low-energy-tail of events which are outside of the energy window used to accept only events which are useful for imaging. This means that in such a situation, the detector has low effective efficiency and low sensitivity and thus is unacceptable for use in imaging applications.
There are several methods known in the art which reduce the low-energy-tail effect caused by the incomplete charge collections of holes due to their low mobility.
A method which makes a detector sensitive only to electron detection is described in an article entitled "Performance of CdZnTe Coplanar-Grid Gamma-Ray Detectors", IEEE Transactions on Nuclear Science, Vol. 43, No. 3, June 1966, by P. N. Luke and E. E. Eissler. According to this method the anode side of the detector includes two coplanar sets of electrodes. One is a grid of anodes and the other is a grid of non-collecting electrodes, and the two grids are displaced relative to each other. When a charge carrier moves in the volume of the detector, it induces current in both grids. As long as its distance from the electrodes, consisting of the two grids, is much larger than the spacing between adjacent individual electrodes of the grids, the induced current is practically equal in both of the grids.
When the charge carrier approaches one of the grids, the induced current in that grid increases. If both sets of grids are maintained at the same positive potential relative to the cathode, the anode-grid, which is the collecting grid, attracts only part of the electrons, while the non-collecting grid collects the remaining part. Accordingly, such a detector is quite inefficient as only part of the photons absorbed by the detector are measured.
Luke et al. overcome this problem of the double-grid technology by negatively biasing the non-collecting grid relative to the anode grid. Consequently, in a situation when a charge carrier is in propinquity to the grid of the non-collecting electrodes, its trajectory is bent towards the anode grid, which has a higher voltage than the non-collecting electrodes, and away from the grid of the non-collecting electrodes. In this case, the induced current in the grid of the non-collecting electrodes changes polarity, until the charge carrier reaches one of the anodes of the anode grid, bringing the total induced charge in the anode to the theoretical maximum. On the other hand, the total induced charge on the grid of the non-collecting electrodes, to which this charge carrier did not get, drops to zero.
In practice the spacing between adjacent individual electrodes of the two interdigitated coplanar grids on the anode side is made much smaller than the thickness of the detector, and thus, in the majority of cases, the current induced by the holes moving along their trajectories towards the cathode, and far away from the anode, causes practically identical charge induction in the two grids. Accordingly, by using a differential CSA for subtracting the input signals of those two grids from each other, the influence of the holes may be practically eliminated, thus eliminating the low-energy-tail created by incomplete charge collection of the holes. At the same time, the detector efficiency is maintained at a good level since some of the electron-trajectories which were originally oriented towards the non-collecting grid, are deflected towards the anode grid.
The above-described method suffers from the following disadvantages:
1. The "non-collecting electrodes" are not really non-collecting electrodes, as they have good ohmic contact with the surface of the detector, so that any electrons which approach this electrode, are collected by it. As a result, a significant number of charge carriers resulting from photon-semiconductor interaction events, end up on the non-collecting electrode and do not reach the anodes. They are thus lost, without having made any contribution to the imaging system, resulting in low efficiency and sensitivity degradation.
2. The electrodes of the coplanar anode grid and of the non-collecting grid are spaced very closely, and are biased with a relatively large potential difference between them, resulting in relatively high leakage current between the anode electrodes and the non-collecting electrodes. Such leakage current introduces excess noise which broadens the energy spectrum and reduces the number of events within the energy window used for imaging, thus reducing the detector efficiency.
3. The system is electronically complicated due to the need for two sets of electrodes, and a set of corresponding differential amplifiers, and thus is not easy to implement in an imaging system having a large number of detector-cells.
4. The signal-to-noise ratio (SNR) is decreased by a factor of 1/.sqroot.2 due to differential amplification of the signal from two electrodes. The energy spectrum is accordingly broader by a factor of .sqroot.2, which degrades the imaging quality.
A modified version of Luke's method is described in International Patent Application Number PCT/US96/15919 by B. Apotovsky et al. and assigned to Digirad Incorporated of San Diego, Calif. This application has been published as International Publication Number WO 97/14060. The inventors use a similar electrode structure to that proposed by Luke, but use the term "control electrode" instead of Luke's "non-collecting electrode". This may well be a more accurate description. As described in Luke's original article, this electrode is biased at a positive potential, and so attracts electrons, but since its potential is lower than the anode potential, part of those electrons are indeed redirected towards the anode without their being collected by the non-collecting control electrode.
The Digirad technology does differ from Luke's method in that differential charge sensitive amplifiers are not used. As a result, the only way to reduce the influence of low mobility holes (for eliminating the low-energy-tailing effect) is to increase the "small pixel effect" by reducing the anode size to a very small spot. The strong small pixel effect makes the detector sensitive only to charge carriers that are moving very close to the anode, typically within a distance of the order of the pixel size. Since the holes move towards the cathode, they will not affect the detector response. Such a small anode has a very poor collection efficiency, and the Digirad technology therefore employs the above mentioned control electrode, which assists in attracting the electrons towards the anode, in order to improve the collection efficiency.
The Digirad variation of Luke's method, therefore suffers from all the disadvantages of Luke's method described above, and from one additional disadvantage. The ratio between the anode area and the area of the control electrode is very small, resulting in even greater charge loss to the control electrode than in Luke's original method. This leads to efficiency and sensitivity degradation even more severe than that encountered in Luke's method.
The charge loss to the control electrode has been numerically calculated and measured by applicants. Both the calculations and the measurements are in agreement and show that a large amount of charge is collected by the control electrode and is lost for imaging applications. This phenomenon is demonstrated in Prior Art FIG. 1. This figure shows Multi Channel Analyzer (MCA) spectra numbers 1 and 2, taken from FIGS. 4 and 9 respectively of the above International Publication Number WO 97/14060. The spectra show the number of counts (events) as a function of channel number (energy). Spectrum 1 was measured using a conventional detector geometry, while spectrum 2 was measured in a similar sized detector cell but using the method of International Publication Number WO 97/14060. A comparison between the two spectra clearly indicates that:
1. The low-energy-tailing effect present in spectrum 1 is indeed eliminated in spectrum 2.
2. The number of events in an energy window of +/- FWHM around the peak (the energy range used for imaging) remains about the same in both of the spectra, or even decreases somewhat in spectrum 2.
This means that the Digirad technology described in International Publication Number WO 97/14060, while successful in eliminating the low-energy-tailing effect, does not convert the events removed from the spectrum low-energy-tail into useful events lying within the energy window used for imaging. Consequently, for such applications as imaging in the field of nuclear medicine, where, in order to reduce the radiation dose needed for producing the image, detector sensitivity and efficiency are of primary importance, the Digirad technology has no advantage over the traditional detectors in prior art use, such as those used for obtaining spectrum 1 in FIG. 1. The main use of the Digirad technology would therefore seem to be in Nuclear Physics applications or X-Ray spectroscopic and analysis methods such as Energy Dispersive-X Ray Fluorescence (ED-XRF), where peak to valley ratio of the detector spectrum is a more important parameter than detector efficiency.
Another method for eliminating the low-energy-tailing effect in the detector spectra, caused by the low-mobility holes, is described in the National Phase Filing of International Patent Application Number PCT/US95/09965, some of the inventors of which are applicants herein. According to this method the detector has at least one injecting electrode having a high quality ohmic contact which serves as the cathode electrode. In this case, as in the previous cases, electrons move quickly towards the anode and practically all of them arrive at the anode. Holes, on the other hand, due to their low mobility and short lifetime, can only move a small distance towards the cathode within the integration time of the CSA, and some of them are even trapped in stationary sites, However, in this case, because of the special characteristic of this detector, namely its strong electron injecting cathode, the charge imbalance created by the hole cloud within the detector induces an injection of electrons from the cathode towards the hole cloud, and annihilate holes by direct recombination. In this way, the incomplete hole trajectory is replaced by a trajectory of an electron moving from the cathode towards the holes. Therefore, the charge collection is compensated and the low-energy-tailing effect is prevented.
The above method is superior to the method of Luke and to that of Digirad Inc., since it achieves the same elimination of the low-energy-tailing effect, without count loss to a non-collecting control electrode. Prior Art FIG. 2 shows an MCA spectrum of a detector constructed according to the method described in PCT/US95/09965, as proposed by applicants. From FIG. 2 it is seen that the low-energy-tailing effect is eliminated. In addition the number of counts actually achieved in this invention is about three times greater than the number of counts measured in a similar detector with non-collecting control electrodes, but without the injecting electrode.
While the method described in PCT/US95/09965 is a marked improvement over both the Luke and the Digirad prior art described above, in that it provides an effective solution to the incomplete hole collection problem, and thus eliminates the low-energy-tailing effect, both it and the other methods discussed above still do not provide a solution to another serious detector problem, namely that of incomplete charge collection caused by surface imperfections and contamination.
Prior art conventional detectors, including those mentioned above, suffer from incomplete charge collection and performance degradation, due inter alia to non-uniformities in the electric field thereacross adjacent to the side walls thereof, and due to leakage current along the surface of the detector side walls. Detector problems arising from non-uniform electric fields, leakage currents, charge-carrier traps, charge-carrier recombination centers, conductive contamination, lattice defects and the energy levels of donors and acceptors inside the band-gap are all related not only to the presence of surface imperfections on the detector side-walls but also to surface imperfections in general, and result in performance degradation of the detector. These problems can exist at any surface of the detector, especially if it is exposed and was not previously passivated or treated by other processes to reduce surface imperfections. The above mentioned surface imperfections lead to incomplete charge collection, excessive noise and performance degradation of detectors, and to the best of the inventors' knowledge, this problem has not been effectively addressed in any prior art detectors.
The following U.S. and foreign patent documents show the provision of an external electrode for various purposes distinct and different from those of the present invention: U.S. Pat. Nos. 5,326,996; 4,926,228; Japanese Kokais 60-218870 and 57-201086 and German Offenlegungsschrift DE 3321921.